The present invention concerns a new iron-based powder. More specifically, the invention concerns a new iron-based powder which is useful for the preparation of soft magnetic materials having improved properties when used both at high and low frequencies. The invention also concerns a new method for the manufacturing of the new iron-based powder.
Iron-based particles have long been used as a base material in the manufacture of structural components by powder-metallurgical methods. The iron-based particles are first moulded in a die under high pressures in order to produce the desired shape. After the moulding step, the structural component usually undergoes a sintering step to impart the necessary strength to the component.
Magnetic core components have also been manufactured by such power-metallurgical methods, but the iron-based particles used in these methods are generally coated with a circumferential layer of insulating material.
Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetised or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to a rapidly varying field, the total energy of the core is reduced by the occurrence of hysteresis losses and/or eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions.
Magnetic core components are often made from laminated sheet steel, but these components are difficult to manufacture to net shape for small intricate parts and experience large core losses at higher frequencies. Application of these lamination-based cores is also limited by the necessity to carry magnetic flux only in the plane of the sheet in order to avoid excessive eddy current losses. Sintered metal powders have been used to replace the laminated steel as the material for the magnetic core component, but these sintered parts also have high core losses and are restricted primarily to direct current (DC) operations.
Research in the powder-metallurgical manufacture of magnetic core components using coated iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties. Desired properties include a high permeability through an extended frequency range, high pressed strength, low core losses and suitability for compression moulding techniques.
When moulding a core component for AC power applications, it is generally required that the iron particles have an electrically insulating coating to decrease core losses.
Different types of insulating coatings which are used for particles of iron are disclosed in the literature.
According to DE 1 291 028 the iron powder is treated with a solution of phosphoric acid and it is subsequently washed and dried. This process is characterised in that the iron powder has a particle size of at most 10 xcexcm and that the powder is treated with chromic acid in addition to the phosphoric acid. The publication does not disclose the magnetic properties for materials prepared by using the iron powder.
Another publication within this field is DE 2 825 235, which discloses an iron powder consisting of particles which are coated with an oxide layer. The particle size is between 0.05 and 0.15 mm and the particles have an oxide coating which, calculated on the particle weight, includes 0.3 to 0.8% by weight of oxygen. The oxide coating can be obtained by heating in air or by chemical oxidation, but no process parameters and no analysis of the coated particles are disclosed. From the examples it can be calculated that the permeabilities obtained are in the range of 30 to 35.
European patent application 434 669 concerns a magnetic powder, wherein an electrically insulating coating separates the magnetic powder particles. The particles have an average particle size of 10-300 xcexcm, and the insulating material which covers each of the particles of the magnetic powder comprises a continuous insulating film having a thickness of 10 xcexcm or less, and this film comprises a metal alkoxide or a decomposition product thereof.
WO 95/29490 discloses insulating layers which are obtained by using a method based on phosphoric acid in water.
Finally, DE patent 3 439 397 discloses iron particles which are electrically insulated by a phosphate coating. This coating could be for example magnesium or zinc phosphate. The insulating phosphate coating should be between 0.1 and 1.5% of the weight of the iron particles. Example 1 of this publication discloses in more detail that this electrically insulating coating is obtained by stirring the iron powder in a solution of 89% of phosphoric acid in acetone. A comparative study between the powder according to the WO-patent application and the powder according to the DE patent has revealed that the insulating layer according to the DE Patent includes considerably more oxygen and phosphorus than the powder according to the WO application.
It has now quite unexpectedly been found that remarkably improved properties can be obtained by using the new low oxygen powder according to the present invention. These properties include strength, density, induction and permeability in combination with lower loss.
The new powder is based on a base powder which essentially consists of pure iron and could be e.g. a commercially available water-atomised iron powder or a sponge iron powder with round, irregular or flat particles. Typical examples of irregular, water-atomised powders which can be used are the powders of the ABC 100 and ASC 100 series available from Hxc3x6ganxc3xa4s AB, Sweden. The particle size of the base powder depends on the intended final use of the powder and is generally less than 200 xcexcm and preferably less than 150 xcexcm. For higher frequencies, particles sizes below 45 xcexcm are preferred. It is furthermore preferred that the majority of the particles of the iron base powder should have a particle size above 10 xcexcm.
According to the invention this base powder is provided with an oxygen coating or barrier, and it is a distinctive new feature that the amount of oxygen of the new powder is only slightly elevated as compared with that of the base powder. More specifically the amount of oxygen in the new powder is at most 0.2%, preferably at most 0.15% by weight higher than in the base powder.
It is believed that the surface structure and composition of the particles is important for the properties of the new powder, and for this reason the new powder has been studied by the ESCA method (cf. xe2x80x9cProceedings of the sixth international conference on X-ray optics and microanalysisxe2x80x9d, University of Tokyo Press, 1972, pp 385-392 and 393-398 or xe2x80x9cSolid state chemistry and its applicationsxe2x80x9d by Anthony R. West, Published by John Wilew and Sons, 1984, p 86 and pp 92-96). According to this method the ratio O:P should be less than 30 and more than 1. Preferably this ratio should be less than 15 and more than 2, and most preferably less than 10 and more than 3.
Another important feature of the surface barrier of the particles of the new powder is the thickness and by using the AES method (cf. The publication xe2x80x9cSolid state chemistry and its applicationsxe2x80x9d referred to above) it has been found that the insulating barrier or coating should be less than 100 nm, preferably less than 70 nm, and most preferably less than 50 nm.
The insulating coating according to the present invention is applied on the base powder by treating the base powder with phosphoric acid in an organic solvent for a period sufficient to obtain the indicated amounts. The concentration of the phosphoric acid in the organic solvent should be considerably lower than the concentration disclosed in the DE patent and vary between 0.5 and 50%, preferably between 0.5 and 20%, and most preferably between 1 and 5%. The new powder can be obtained by spraying the phosphoric acid solution on the base powder for a period sufficient to obtain the levels indicated above. The concentration of the phosphoric acid should preferably be less than 10 and most preferably less than 5% by weight.
The new iron-based powder according to the invention can be combined with a lubricant in an amount of 0.1 to 1.0% by weight and optionally an organic thermosetting or thermoplastic resin before the compacting step. Representative examples of lubricants are Kenolube(copyright),H wax, EBS and stearates, such as zinc stearate. The organic resin could be selected from the group consisting of Peracit, Ultem.
The compacting could be carried out at conventional pressures up to 1000 Mpa and the compacting could be carried out both at ambient and elevated temperature.